Our planet's surface area is covered primarily by water. Aquatic water systems like oceans, rivers, bays and estuaries, lakes, and ponds cover over two-thirds of the Earth's surface. Pollutants can contaminate these surface water sources that harm numerous plant and animal species. But, they also frequently seep directly into the ground and pollute the aquifers and other ground water systems below.
Ground water naturally contains metallic and non-metallic ions that slowly dissolve from soil particles, sediments, and rocks as the water travels above and underground. Metallic ions typically appear only in trace amounts in natural groundwater (e.g., less than 1.0 mg/L=1.0 ppm). Major cation constituents like sodium, calcium, and magnesium and major anion constituents like bicarbonate, sulfate, chloride, and silica (e.g., 1.0-1000 mg/L=1-1000 ppm) also naturally appear in such groundwater.
But human activities also impact ground water, introducing pollutants in the process to elevate the natural contamination level of the ground water. Heavy metal cation contaminants like arsenic, lead, mercury, cadmium, iron, aluminum, antimony, chromium, cobalt, copper, manganese, nickel, uranium, vanadium, and zinc become toxic when they are not metabolized by the body, and accumulate in the soft tissues, posing significant health problems. Sulfate anion contaminants constitute another widespread contaminant. Sulfates and sulfuric acid products are commonly used in the manufacture of fertilizers, chemicals, dyes, glass, paper, soaps, textiles, fungicides, insecticides, astringents, and emetics. They are likewise employed by the mining, wood pulp paper, metal, and plating industries, as well as for sewage treatment and leather processing. These sulfate compounds may be discharged into water effluent streams emanating from the industrial plants. The mining industry excavates the earth for coal and precious metals, turning deeply embedded rocks containing sulfur compounds up to the earth's surface with the resulting sulfates winding up within the water runoff that enters nearby rivers and lakes. Such sulfates react with organic carbon and water to produce hydrogen sulfide (H2S) and carbonic acid. The presence of H2S in drinking water produces a “rotten egg” odor. Indeed, sulfate compounds contained in drinking water can be tasted at concentrations as low as 250 mg/L.
Thus, government regulations often impose discharge limits between 250 mg/L and 2000 mg/L for industrial wastewaters containing high sulfate concentrations. At levels above this 250 mg/L threshold, sulfates can cause diarrhea and resulting dehydration. High sulfate levels in drinking water can also cause fluid and weight loss in animals.
It is therefore necessary for miners and manufacturers to treat these industrial wastewater streams to reduce these heavy sulfate concentrations to acceptable levels before they are introduced into water streams and water bodies that are subject to environmental government laws and regulations. Several technologies are available for removing sulfates from water. The most common method used is a chemical process in which hydrated lime (Ca(OH)2) is added to the wastewater stream to react with sulfate compounds in the water to precipitate calcium sulfate (CaSO4). Such calcium sulfate hydrates to become common mineral gypsum which is used in wall board. But, because calcium sulfate has a solubility of approximately 2000 mg/L as sulfate, this method does not provide effective sulfate reduction below this 2000 mg/L concentration level.
Reverse osmosis represents a mechanical process for removing contaminants from wastewater streams that can produce sulfate levels below 2000 mg/L. High-pressure commercial reverse osmosis units can reduce sulfates contained in wastewaters by 95%. However, this technology requires costly equipment, and produces byproduct streams that must in turn be treated and handled.
A “solution” represents a mixture of two or more individual substances that cannot be separated by a mechanical means, such as filtration. For example, a liquid solution occurs when a liquid, solid, or gas solute is dissolved in a liquid solvent. The liquid solution constitutes an aqueous solution if the solvent is water. Wastewater streams very often constitute aqueous solutions containing one or more contaminants.
Another well-known wastewater treatment method is “adsorption” in which the toxins and contaminants contained in the dissolved, aqueous phase in the wastewater are transferred to the surface of solid sorption media. Such a medium that is high in carbon may be activated whether by chemical, physical, or thermal means to increase the surface area and create porosity for the resulting toxins and contaminants that are sorbed onto active sites on the media surface. However, such sorption media that are particularly useful for removing toxic cations from wastewaters do not constitute ion exchange media.
Ion exchange is a separation process widely used in the food and beverage, hydrometallurgical, metals finishing, chemical and petrochemical, pharmaceutical, sugar and sweeteners, ground and potable water, nuclear, softening and industrial water, semiconductors, power, and many other industries. Aqueous and other ion-containing solutions can be purified, separated, and decontaminated by swapping targeted ions contained in the solution with substitute ions typically provided by ion exchange resins or other substrates.
But ion exchange is also a proven technology for removing dissolved anions or other impurities from these wastewater streams. It represents a reversible process in which the ionized compound or element changes place with another ionized compound or element on the surface of a medium like an ion exchange resin. For example, high levels of sulfates up to 1500 mg/L can also be removed by an ion exchanger sold by Rain Dance Water Systems that uses a specially designed resin to replace the sulfates with harmless chlorine in the water.
“Cost-Effective Sulfate Removal” (“CESR”) is another treatment system for removing sulfate that was commercialized by Hydrometrics, Inc. of Helena, Mont. It is a chemical treatment process that treats lime-treated water to precipitate sulfate as a nearly insoluble calcium-alumina-sulfate compound known as “ettringite.” But this system also requires costly equipment and produces a substantial amount of byproduct streams that must in turn be treated and handled for proper disposal.
Ion exchange can produce high-purity water (including softening, deionizing, water recycling, and removal of heavy metals) from the wastewater. In a familiar example to many readers, a cation exchange-based water softener works by passing hard water naturally containing an abundance of calcium and magnesium cations through a volume of resin beads containing sodium ions on their active sites. During contact, the calcium and magnesium cations will preferentially migrate out of solution to the active sites on the resin, being replaced in solution by the available sodium ions. This process reaches equilibrium with a much lower concentration of calcium and magnesium cations in solution, thereby “softening” the water. The resin can be recharged periodically by reacting it with a solution containing a high concentration of sodium ions, such as a sodium chloride solution. The calcium and magnesium cations accumulated on the resin will migrate off it, being replaced by the sodium ions from the salt solution until a new equilibrium state is reached.
In the case of anion exchangers, the media exchanges one negatively charged ion for another. For example, the functional groups on the media may be in the chloride form which can be exchanged for sulfate ions in solution. As a result, sulfate is removed from the water and chloride is added to the water. In the case of a weak anion exchanger, a NH2 functional group is typically attached to the polymer backbone of the resin bead. It reacts with an acid like HBr contained in wastewater as follows:
thereby removing the acid from the wastewater and attaching it to the resin beads via a chemical bond. This NH2 functional group pre-attached to the polymer backbone represents an amine derivative. Such a weak anion exchange medium is generally effective for adsorbing sulfates from wastewaters under acidic conditions. The resin beads can be regenerated by treating them with NaOH or Ca(OH)2.
By contrast, a strong anion exchanger generally constitutes a polymer backbone to which is pre-attached an amine salt in a charged state. It will react directly with charged anions contained in the wastewater to exchange the charged anion on the resin into solution for the charged anion contaminant as follows:
This type of strong anion exchange resin is effective at adsorbing sulfate contaminants under less acidic conditions and therefore over a broader pH range than is the case for weak anion exchangers. Such strong anion exchangers may be regenerated by a salt like NaCl to liberate the accumulated Br− anion contaminant.
Amines represent a useful functional group for converting a polymer resin into a useful anion exchange medium. Macroporous weak base anion exchange resins are characterized by tertiary amine groups that are attached to a cross-linked polystyrene matrix. The matrix is commonly prepared using divinylbenzene as a cross-linking agent in the presence of a linear polymer like polystyrene in order to introduce porosity into the formed macroporous matrix. This matrix is then chlorormethylated by a reaction with paraformaldehyde, methanol, hydrochloric acid, or chlorosulfonic acid in the presence of a Friedel-Craft catalyst such as aluminum chloride, zinc chloride, or ferric chloride. The resulting chloromethylated resin beads are washed with water and aminated with dimethyl amine. This type of weak base anion exchanger exhibits excellent adsorption and desorption properties for removal of mineral acids, organic matter, chromates, or formic acid. It can be regenerated by alkalis such as sodium carbonate or ammonium hydroxide. See “Weak Base Anion Exchange Resin: Simplification of Amination Process and Control on SBC, Presented at Ion Exchange Advances, SCI Conference IEX '92 (Churchill College, Cambridge, UK Jul. 12-17, 1992)(M. J. Slater, Editor; Elsevier Applied Science).
Synthetic ion exchange resins are typically used within ion exchange processes. These synthetic resins commonly are formed of small 0.03-2.0 mm beads made from an organic polymer substrate, such as cross-linked styrene and divinylbenzene copolymers. Moreover, these resin beads will feature a highly developed structure of pores on the surface of the resin, which provide the sites for exchanging ions. These resin beads can be converted to cation-exchange resins through sulfonation, or to anion-exchange resins through amination of chloromethylated derivatives.
Thus, U.S. Pat. No. 4,177,331 issued to Amick discloses a sulfone cross-linked polystyrene resin that is useful as a weak or strong anion exchanger. A linear polystyrene is cross-linked with a sulfonating reagent like chlorosulfonic acid, sulfur trioxide, sulfuric acid, and a boron compound such as boric acid or boron oxide. The resulting reaction is controlled to favor the formation of a sulfone cross-linked sulfonyl chloride intermediate. The intermediate may then be converted into either the weak or strong anion exchanger.
U.S. Pat. No. 5,726,210 issued to Teraue at al. teaches the production of an anion exchange resin comprising reacting an aromatic cross-linked haloakyl-containing copolymer with an amine in the presence of water and a water-soluble inorganic salt. Preferred examples of the amine include trimethylamine, triethylamine, dimethylamine, diethylamine, and diethyl ethanolamine.
U.S. Pat. No. 8,846,773 issued to Fukui et al. illustrates an anion exchange resin prepared by polymerizing divinylbenezene with styrene, alkylating the resulting polymer with chloromethyl methyl ether using zinc chloride as a catalyst, and further reacting the compound with trimethylamine. The resulting cross-linked copolymer is then sulfonated to form a sulfonated cross-lined copolymer.
Hexamethylenetetramine (commonly known as “urotropin”) represents a useful amine-based reagent that is readily obtainable from ammonia and formaldehyde. It is soluble in water, chloroform, ethanol, and some other organic solvents. It also remains stable at elevated temperatures with a symmetrical adamantine-like structure.
See N. Blazevic, D. Kolbakh, B. Belin, V. Sunjic & F. Kajfez, “Hexamethylenetetramine, A Versatile Reagent in Organic Synthesis” (Georg Theime Publishers; Issue 3, pp. 161-76) (1979).
Moreover, resin-like compounds may be formed from the reaction of phenol with hexamethyltetramine. Although Mortimer Harvey and L. H. Baekland demonstrated the presence of nitrogen in the resulting compound, they failed to show the amine nature of the nitrogen. Moreover, no evidence was provided that this nitrogen behaved like a weak anion exchanger. M. Harvey & L. H. Baekland, “Further Studies of Phenolic Hexamethylenetetramine Compounds”, Journal of Industrial & Engineering Chemistry (vol. 13, no. 2, pp. 135-41) (1921).
U.S. Pat. No. 4,200,706 issued to Starks discloses the curing of cross-linked phenol-formaldehyde resoles or novalacs at low temperatures and pressures. Divinylbenzene is used as the cross-linking agent along with a small amount of acid catalyst. Normally, the phenol-formaldehyde novalacs require the addition of cross-linking agent. Hexamethylenetetramine is the most commonly used cross-linking agent. Solid hexamethylenetetramine is mixed with a novalac to produce a syrup which will cure upon heating. Such novalacs, when heated at temperatures up to 140° C. form bis- and tri-hydroxybenyl amines. The bis- and tri-hydroxybenzyl amines reacting with an excess of phenol, eliminate nitrogen to produce methylene bridges. At temperatures of from 160° C. temperatures amine linkages undergo further reactions leading to decomposition which produces xanthene and methyl phenols along with further ammonia and methylamine. This procedure is explained in greater detail in the Lin-Gibson, S.; Riffle, J. S., Chemistry and Properties of Phenolic Resins and Networks. In Synthetic Methods in Step-Growth Polymers, John Wiley & Sons, Inc.: 2003; pp 375-430. Thus, it has been recognized that reaction novalacs with hexamethylenetetramine can produce benzyl amines. However, these authors did not disclose the production of weak anion exchange resin using reaction of novalacs with hexamethylenetetramine.
This process for producing synthetic resins is expensive. The resin beads are also highly susceptible to “fouling.” While soluble organic acids and bases removed by the synthetic ion-exchange resin are shed during regeneration, non-ionic organic materials, oils, greases, and suspended solids also removed from the water tend to remain on the surface of the resin bead. Foulants can form rapidly on the resin, and can significantly hinder performance of the ion-exchange system. Cationic polymers and other high molecular weight cationic organics are particularly troublesome at any concentration. For certain types of resins, even one ppm suspended solids can cause significant fouling of the resin beads over time. Thus, a prefiltration unit in the form of activated carbon or other separation material may need to be positioned upstream of the ion-exchange unit to remove these organic contaminants before the wastewater is passed through the ion exchange resin, further complicating the water treatment process and its costs. The costs associated with this pretreatment can be substantial.
Additionally, resins require regeneration once the ion-exchange sites have been exhausted, for example, as feed water flows through a bed. During regeneration of an ion exchange resin, anions that were previously adsorbed from the wastewater flow are replaced on the resin beads by hydroxide ions. A step known as “backwash” is often employed during regeneration, so that any organic contaminant buildup in the resin can be relieved, thereby allowing free flow of the wastewater through the resin beads. But, chemically-regenerated ion-exchange processes known in the art tend to use excessive amounts of regeneration chemicals, which require periodic and even on-going treatment, as well as safe disposal of the chemical waste. These processes can be complex and expensive to operate.
Peat-Based Sorption Media
It would therefore be desirable to produce an anion exchange medium from a natural, organic material. However, a balance must be struck between the physical and chemical integrity of the form of the anion exchange medium versus the ability of the medium to serve as an anion exchanger.
Phenol-containing organic starting material like peat inherently possesses cation-exchange and adsorbent characteristics. Peat is composed mainly of marshland vegetation, trees, grasses, fungi, as well as other types of residual organic material such as insects and animal remains, and is inhibited from decaying fully by acidic and anaerobic conditions. It is also abundant in many places in the world. For example, 15% of Minnesota is covered by valuable peat resources, comprising 35% of the total peat deposits found in the lower 48 states in the U.S.
Pellets made from peat are known within the industry. For example, U.S. Pat. No. 6,455,149 issued to Hagen et al. discloses a process for producing peat pellets from an admixture of peat moss, pH adjusting agent, wetting agent, and other processing additives. The resulting granules can be easily broadcast spread on the ground, and returned to their original peat moss form upon wetting to act as a fertilizer. U.S. Pat. No. 3,307,934 issued to Palmer, et al. shows another fertilizer product containing peat, and water-soluble inorganic fertilizer salt like diammonium phosphate, sulfate of potash, or urea. However, neither of these products are capable of being used as an anion exchange medium.
Russian Patent No. 2,116,128 issued to Valeriy Ivanovych Ostretsov teaches a process for producing a peat sorbent useful for removing oil spills from solid and water surfaces. The peat material is dried from 60% moisture to 23-25% moisture, and then compressed at 14-15 MPa pressure into briquettes. Next, these peat briquettes are heated at 250-280° C. without the use of additional hydrophobic chemicals and without air. The humic and bitumen fractions within the peat mobilize to the surface of the peat briquettes to produce a natural hydrophobic coating. This hydrophobic coating is necessary for the peat briquettes to be able to soak up oil. Ostretsov also reduces the moisture of his heat-treated peat briquettes all the way down to 2.5-10% wt. moisture. This significant water reduction assists with the hydrophobic coating formation and frees up the pores in the peat material so that they are available to soak up oil. See also Russian Patent No. 2,173,578 also issued to Ostretsov.
It is also known in the wastewater treatment industry to use pellets made from peat or other natural organic materials as a pollution filtering medium. For instance, U.S. Pat. No. 5,624,576 issued to Lenhart et al. illustrates pellets made from leaf compost, which are then employed to remove pollutants from storm water. U.S. Pat. No. 6,143,692 issued to Sanjay et al. discloses an adsorbent made from cross-linked solubilized humic acid, which can be employed for removing heavy metals from water solutions. See also U.S. Pat. No. 6,998,038 issued to Howard; U.S. Pat. No. 6,287,496 issued to Lownds; U.S. Pat. No. 5,578,547 issued to Summers, Jr. et al.; and U.S. Pat. No. 5,602,071 issued to Summers, Jr. et al. However, all of these prior art references disclose sorption media useful for sorbing heavy metals or other cations—not anion exchange media useful for removing anions from wastewater or other aqueous solutions.
Peat as a Cation-Exchange Media
Various efforts have been made to prepare cation-exchange mediums from peat starting material which is activated and, in some cases, chemically modified before the chemical activation step. These sorbents are designed to remove cations like cadmium, zinc, copper, etc. from wastewater. Cation exchange media generally use carboxylic acid (COOH) groups or SO3− groups attached to the peat substrate to attach and therefore remove the cations from the wastewater. For instance, U.S. Pat. No. 4,778,602 issued to Allen, III teaches a multi-functional filtering medium consisting of highly humified peat which is treated with an alkaline solution to hydrolyze the humic and fulvic acid fractions contained therein. Next, the peat product is treated with a quaternary amine solution to precipitate out the humic and fulvic acid fractions from the peat. After filtering the dried the peat cake, nitric acid or sulfuric acid is added to neutralize the amine to chemically modify the peat to increase its cation exchange sites by either adding SO3− groups to the peat surface structure, or to oxidize the organic carbon to improve the cation exchange capacity. Finally, the peat residue may be treated by a semi-coking process step at 200-1000° C. at a 40 psi pressure, thereby allowing carbonization of peat residue. But, this will actually destroy the carbon fibers. The enhanced cation exchange capacity is also aided by destruction of the carbon fibers via the semi-coking (pyrolyzation) step.
U.S. Pat. No. 6,042,743 issued to Clemenson discloses a method for processing peat for use in contaminated water treatment. Clemenson mixes raw peat with heated sulfuric acid to produce a sulfonated peat slurry. After cooling and drying the slurry admixture to a 60-70% moisture content, he adds a binder like bentonite clay to coagulate the acidic peat slurry, extrudes pellets, and then bakes the sulfonated peat pellets in an oven at 480-540° C. His procedure will add sulfonic groups (—SO3−H) to the resulting peat granules, thereby increasing cation-exchange capacity. See also U.S. Pat. No. 6,429,171 issued to Clemenson.
In yet another example, U.S. Pat. No. 5,314,638 issued to Morine discloses a chemically-modified peat product that can be used as a cation-exchange material. This peat material is air dried and milled to a size of one mm or less; hydrolyzed in an aqueous hydrochloric acid solution to remove the soluble components (sulfuric acid and nitric acid may also be used); further treated in an extractor with 2-propanol/toluene solvent to remove the solvent-soluble bitumen; dried to remove the residual solvent; and then immersed in a hot concentrated sulfuric acid bath at 100-200° C. for 1-4 hours. The hot sulfuric acid bath process step comprises chemical modification in which the sulfuric acid reacts with the peat fibers to add sulfonic groups (SO3−H) to its surface. These acid groups attract metals via a cation exchange mechanism. But in all of these cases, the prior art products are acting as cation exchange media for removing heavy metals or other cations—not anion exchange media useful for removing anions from wastewater or other aqueous solutions.
Challenges Faced by Peat and Other Natural Organic Materials as Anion Exchange Media
But, the large body of available research illustrates the underlying shortcomings for natural peat for use as an anion exchanger. First, peat starts out with zero anion exchange capacity. The peat-based cation exchange media known in the prior art will not work to remove anions from wastewaters or other aqueous solutions.
Second, in its natural form, peat has low mechanical strength, tends to shrink and swell, and does not allow for hydraulic loading. Naturally-occurring organic ion exchange media are unstable outside a moderately neutral pH range. Thus, any peat-based anion exchanger must be modified to increase its physical stability, particularly within high-pH conditions.
Third, such natural organic ion exchange media like peat tend to be prone to excessive swelling and peptizing, and tend to leach naturally occurring heavy metals from the substrate material into the treated wastewater solution.
Fourth, prior art steps commonly applied to peat and other organic materials like pyrolysis can cause these materials to lose their ion-exchange capacity. Carbonization may cause considerable shrinkage and weight loss of the materials, as well as loss of naturally-occurring phenolic compounds.
While the processes known in the art for the preparation of sorption material sourced from natural solid organic material like activated carbon have been useful for certain limited applications, for many anion exchangers it will be necessary to introduce new functional groups to the surface of the natural materials. Also, once an anion exchange media has been used and the functional groups are exhausted, it is necessary to follow the sorption step with a regeneration step where bases like Ca(OH)2 or NaOH are used to remove the anions from surfaces of the spent anion exchanger. This step requires a physical stability of an anion exchanger at high pH environment. It is therefore necessary to develop a low-cost process for producing an anion-exchange media sourced from natural organic starting material that exhibits good natural anion-exchange capacity and good physical stability at high pH, so that the medium can be utilized in a wider range of end-use applications, including the removal of anions like sulfates from industrial wastewaters.
But peat provides another chemical challenge to the production of anion exchange medium. The prior art polymer-based weak anion exchangers discussed above added amine functional groups to the polymer backbone in the absence of any phenol groups being present to participate in the resulting reaction. However, peat naturally contains phenol groups within the humic acid, fulvic acid, and lignin fractions of the peat material, and it would be desirable to take advantage of those phenolic structures to add amine functional groups to the peat matrix.
The well-known “Duff Reaction” involves the treatment of a phenol with hexamethylenetetramine in an acidic media provided by, e.g., acetic acid, trifluroroacetic acid, hydrochloric acid, or sulfuric acid, followed by hydrolysis in order to produce a desired salicylaldehyde as follows.

The advantage of this Duff Reaction formulation is that it combines inexpensive and easily available reagents with a tolerance for traces of moisture. Moreover, it is comparative with a wide variety of functional groups represented by R. See Y. Ogata, A. Kawasaki & F. Sugiura, “Kinetics and Mechanism of the Duff Reaction”, Tetrahedron (vol. 24, pp. 5001-5010) (1968); I. S. Belostotskaya, N. L. Komissarova, T. I. Prokof'eva, L. N. Kurkovskaya & V. B. Voleva, “New Opportunities for Duff Reaction”, Russian Journal of Organic Chemistry (vol. 41, no. 1, pp. 703-06) (2005).
However, the product resulting from this Duff Reaction contains an aldehyde functional group instead of the amine functional group that is desirable for a weak anion exchange medium, despite the fact that hexamethylenetetramine was reacted with the phenol-based starting substrate. The researchers in N. Grimblat, A. Sarotti, T. Kaufman & S. Simonetti, “A Theoretical Study of the Duff Reaction: Insights Into Its Selectivity,” Organic & Biomolecular Chemistry, (vol. 14, pp. 10496-10501)(2016) propose the following reaction mechanism:

As can be seen from the mechanisms of the Duff Reactions in Equations I, II, and III, an amine can be formed as an intermediate in the Duff Reaction. However, in the normal course of the reaction, the amine ultimately gives rise to the aldehyde moiety. The present invention capitalizes on the discovery that the Duff Reaction can be halted at the amine intermediate step without proceeding to the aldehyde.